Cutting elements, such as shear cutters for rock bits, for example, typically have a substrate to which is bonded an ultra hard material layer forming a cutting layer (also referred to as a cutting table). The substrate is generally made from a fully dense cemented tungsten carbide-cobalt (WC—Co) material (sometimes referred to simply as “carbide”), while the cutting layer is made from a polycrystalline ultra-hard material, such as polycrystalline diamond (“PCD”) or polycrystalline cubic boron nitride (“PCBN”). Ultra-hard material particles and a binder are positioned over the substrate in a mold commonly referred to as a can and are subjected to high-pressure/high temperature conditions (“HPHT”). The HPHT process simultaneously accomplishes two critical functions: a) the ultra-hard material particles are sintered forming a polycrystalline ultra hard material layer, and b) the ultra-hard material layer and the substrate are metallurgically joined along a common interface.
Common problems that plague cutting elements and specifically cutting elements having an ultra-hard cutting layer, such as PCD or a PCBN, layer bonded on a solid carbide substrate, are chipping, spalling, partial fracturing, cracking or exfoliation of the cutting table. These problems result in the early failure of the cutting layer and thus, in a shorter operating life for the cutting element. Typically, these problems may be the result of residual stresses generated on the ultra-hard material layer. Residual stresses are thought to be generated by the coefficient of thermal expansion (CTE) mismatch between the substrate and the ultra hard material layer.
To address the residual stress problem, a number of engineering fixes have been attempted by making the interface between the substrate and the ultra hard material layer discrete. For example, to reduce the residual stresses formed on the interface between the substrate and the cutting layer and to enhance the delamination resistance of the cutting layer, irregularities are sometimes formed or machined onto the interface of the substrate which interfaces with the ultra hard material layer, forming a non-uniform interface between the substrate and the ultra hard material layer. Moreover, transition layers may be incorporated in some cutting elements between the substrate and the ultra hard material layer. The transition layers typically have properties, including a CTE, which are intermediate between the properties of the substrate and the cutting layer, thus, reducing the residual stresses generated by the CTE between the substrate and the ultra hard material layer.